Mine is a condo too. I got one heater that is installed at my aircon ledge and it supplies hot water to the whole apartment. This is not common as most condos only have a short copper pipe from mixer to the plaster ceiling above in each bathroom. So you will need to have one heater for each bathroom. But if you got two bathrooms adjacent to each other, you can get away with just one heater supplying to both bathrooms.

You worry too much. Totally unnecessary. If you worry about electrocution, just turn off your heater before showering. If you worry about explosion because of too much pressure, dont turn it on 24x7, just turn it on 10mins before showers and off during showers. No power means no heat. No heat means no pressure will build up. There is already a temperature sensor that will cut off electricity to the heater when it gets too hot. And a pressure release valve to release pressure. Both have to fail before anything bad happens. If you practice to turn off your heater off, then there is a total of 3 fail-safe

If you want a centralized heater like what I am using, then you must be prepare to hack and install copper pipes from heater to all the locations you need hot water. Btw mine is turn on 24x7

safest is no heater - true story.

on a serious note - i have use gas type before.
they last practically forever.
down side is run out of gas = no heater.
so need to keep spare gas tank.